It has so far been known that in producing a rubber composition containing a filler such as carbon black and the like in the field of rubber production, a wet master batch is used in order to enhance a processability of the rubber composition and a dispersibility of the filler.
A wet master batch is produced generally by passing through an optional rubber latex solution preparation step for adding optionally a surfactant, water and the like to a latex of a natural rubber and a synthetic rubber to prepare a rubber latex solution, a filler slurry preparation step for dispersing a filler such as carbon black and the like in a solvent such as water to prepare a slurry solution, a mixing and coagulating step for continuously mixing and coagulating the rubber latex solution and the filler slurry which are prepared in the above steps to form a coagulated liquid containing a coagulated matter, a solid-liquid separating step for separating and taking out only the coagulated matter from the coagulated liquid formed, a washing step for washing the coagulated matter separated to remove impurities such as the coagulant and the like adhered thereon or contained therein from the coagulated matter, a dehydrating step for removing a moisture remaining in an inside vacant of the coagulated matter washed, a drying step for drying the coagulated matter dehydrated, and an optional molding step for molding the coagulated matter dried into a desired form such as a particular form, a sheet form and the like to form the wet master batch of the product.
When using a wet master batch, it is characterized by that obtained is a rubber composition which is excellent in a dispersibility of a filler and excellent in rubber properties such as a processability and a reinforcing property as compared with conventional rubber compositions obtained by mixing components in a solid state. Use of a rubber composition prepared by using a wet master batch for raw material makes it possible to produce, for example, rubber products such as tires which are reduced in a rolling resistance and are excellent in a fatigue resistance and an abrasion resistance.
On the other hand, in producing the wet master batch, the filler has to be sufficiently dispersed in the stage of the wet master batch in order to enhance the physical properties of the final vulcanized rubber. The step requiring dispersion has so far been extended over two stages, and the dispersion is secured via a mixing step and a kneading step (biaxial extruding step). However, the desirable equipment is to achieve the dispersion only by the kneading step.
In the light of an excellent dispersibility as compared with a rubber composition prepared by mixing components in a solid state, it is important that carbon black and a rubber latex solution are sufficiently mixed in the stage of a mixing and coagulating step for mixing and coagulating a latex and a carbon slurry.
A method for enhancing the physical properties of the final vulcanized rubber by optimizing a particle size distribution of carbon black taking note of a dispersion state of a carbon black slurry solution which is a raw material is reported as a method for sufficiently mixing carbon black and a rubber latex solution.
Known are, for example, 1) a method for producing a wet master batch, comprising a step for mixing a slurry solution prepared by dispersing a filler such as carbon black in advance in water with a natural rubber latex, wherein a particle size distribution of the filler in the slurry solution is controlled to 30 μm or less in terms of a 90 volume % particle diameter (D90) (refer to, for example, a patent document 1), and
2) a natural rubber wet master batch composition obtained by, for example, a producing method in which a slurry solution prepared by dispersing carbon black in advance in water is allowed to collide with a natural rubber latex at a high pressure to mix them (refer to, for example, a patent document 2).
Given is a method in which an optional mixer is used to carry out kneading and dehydrating while heating a filler-containing rubber coagulated matter in the methods for producing a rubber wet master batch in the patent documents 1 and 2 described above. In the above kneading and dehydrating method, the more the mechanical energy in kneading is elevated, the more the dispersibility of the rubber wet master batch obtained after dehydration can be enhanced.
However, if a heat quantity and/or a mechanical energy applied in dehydration are increased, caused is the problem that a polymer molecular chain of the resulting rubber wet master batch is broken to deteriorate the rubber characteristics of the rubber composition finally obtained in a certain case.
On the other hand, known as a method for producing a rubber wet master batch other than the methods described above are, for example,
3) a method for producing a rubber chemical-containing master batch, characterized by drying and kneading a rubber wet master batch containing a rubber component and a filler by means of a biaxial extruder which is a first kneader and then further mixing it with a rubber chemical by means of a Banbury mixer which is a second kneader (refer to, for example, a patent document 3), and4) a method for producing an elastomer composite material by carrying out a mastication step for a rubber coagulated matter obtained after a dehydrating step while applying a mechanical energy of at least about 0.3 MJ/kg (about 249 W/kg) and continuously carrying out an additional mastication step while applying a mechanical energy of about 0.9 MJ/kg (about 83 W/kg) (refer to, for example, a patent document 4).
However, the producing method of the patent document 3 described above involves the problem that since the rubber wet master batch is produced by drying and kneading the filler-containing rubber coagulated matter by means of the biaxial extruder, a mechanical energy applied to the rubber wet master batch is increased and that the rubber component is likely to be deteriorated.
Also, the problem that a mechanical energy applied to the rubber wet master batch is increased as well in the producing method of the patent document 4 described above and tends to deteriorate the rubber component.